1. Field of the Invention
The present invention relates generally to testing methods for lifetime prediction of gas lubricated interfaces. The present invention relates more particularly, but not by way of limitation, to magnetic hard disk drive devices, and to the experimental acceleration of surface wear in the interfaces between the recording heads and the recording media, in magnetic hard disk drive devices.
2. Description of the Prior Art
In a magnetic hard disk drive device, data is written and read in concentric circular tracks on the surfaces of a stack of co-rotating disks. Magnetic recording heads, which are mechanically pressed against the disk surfaces by a "preload", read and write the data. The preload arises from the deflection, during disk drive assembly, of a supporting structure known as the Head Gimbal Assembly (HGA). In typical operation, heads are not in continuous contact with the disk surfaces, because of the intentional development of a gas separation layer, or "gas bearing", which acts as hydrodynamic lubrication. This gas bearing is comprised of a thin layer, or partial layer, of gas, entrained between the surfaces due to the tangential motion of the disk relative to the head, in the presence of the gaseous atmosphere present in the disk drive. This atmosphere is typically air. Head designers attempt to minimize the gas bearing thickness, or "fly height", since this minimizes bit-to-bit and track-to-track spacing on the disk, and thereby maximizes data storage capacity.
Many modem disk drive device designs have gas bearings that are so thin that the average minimum spacing between the recording head surface, and the disk surface, is just a few tens of nanometers, during normal operation. An example of a recording head design for such an application is given by Leung, et. al. in U.S. Pat. No. 5,473,485. In such designs, the recording head frequently contacts asperities on the disk surface during normal operation (ref.: Bogy, et. al., 1993), and the gas bearing does not offset all of the preload. The advantage of such designs is a higher data storage density on the disk surface. A disadvantage of such designs is that friction and wear of the head disk interface is no longer exclusive to periods of starting and stopping of the device, but can also occur in significant proportions during periods of normal operation.
The aforementioned disadvantage severely downgrades the accuracy of many conventional methods of experimental interface lifetime prediction. Three such methods are Pin On Disk testing (POD), Constant Speed Drag testing (CSD), and Contact Start Stop (CSS) testing. The patented method of Kobayashi, et. al. (U.S. Pat. No. 4,966,030) is an example of POD. The patented method of Eltoukhy, et. al. (U.S. Pat. No. 5,074,983) provides for correlation with CSS results. The patented method of Chen (U.S. Pat. No. 5,038,625) combines POD and CSS. Since POD, CSS, and CSD, accelerate wear through elimination or alteration of the aforementioned gas bearing, they are incapable of investigating the effect of medial gas bearing characteristics on tribological phenomena (ref.: Azarian and Bauer, 1993). CSS can only consider transient gas bearing characteristics during the periods of device start-up or shut-down. Furthermore, CSS and CSD testing is performed at disk speeds which are much lower than the design speed of the disk drive. This alters the energy associated with asperity contacts in the interface. Therefore, CSD & CSS testing can lead to inaccurate lifetime predictions for disk drive devices which have a very small typical gas beating thickness during operation. They are also of very limited utility to the gas bearing designer.
For new disk drive designs, in which significant interface wear may occur in the presence of the fully developed gas bearing, more representative wear tests are needed to predict device lifetime. Several methods have been proposed to satisfy this need.
Peck, et. al. (1993), suggested acceleration of wear by increasing the preload on the head through the action of a pneumatic cylinder acting on the HGA. However, this method is not representative of the true wear situation in a disk drive since: the gas bearing stiffness and gas bearing resonance frequencies are altered. Furthermore, the boundary conditions on the HGA are also altered, giving rise to non-representative HGA resonance and compliance properties.
Another conventionally proposed method for accelerating wear, of the magnetic head disk interface, relies on the reduction of ambient pressure. However, this method also has the disadvantage that gas bearing properties and resonance frequencies are altered. Furthermore, tribochemical interactions at the interface may be altered due to changes in the concentration of available oxygen and water vapor.
Another conventionally proposed method increases wear rate by means of introduction of particulate contamination into the interface. However, this can only be a representative test if it is known, in advance, that three-body abrasive wear is the dominant failure mode in the actual disk drive. Furthermore, the properties of the wear particles which are generated naturally by the interface are not constant, since they oxidize over time and increase in hardness. Consequently, this method requires, in advance of testing, possession of much of the knowledge that testing is being accomplished to acquire.
The patented method of Chen (U.S. Pat. No. 4,416,144) provides for denting of the disk media at periodic intervals, and shows that disk topographical disturbances can excite gas bearing resonance. However, this method radically changes the disk topography. Asperity heights, thicknesses, and spatial distributions, resulting from this method, do not represent those found under operating conditions in the disk drive device, and so the accuracy of predictions based on this method is reduced. Furthermore, use of this method to compare different gas bearing designs, which have different minimum gas bearing thicknesses, necessitates the use of dents of different height. This does not facilitate the comparison of different gas beating designs, as used on the same disk surface.
Other related prior art has had the object of reducing, rather than increasing, interface wear rate. The patented method of Bryant, et. al. (U.S. Pat. No. 5,466,979) uses non-uniform vibration, resulting from induced surface resonance, to reduce the wear rate of surfaces that are maintaining continuous sliding contact. However, this method is not applicable to magnetic head disk interfaces which have a gas bearing, due to a condition that the surfaces stay in continuous contact. Even if the method of Bryant et. al., allowed the existence of a gas bearing, gas bearing resonance excitation would not be allowed by the method, since, in that case, the condition that vibration be non-uniform over a length scale corresponding with "islands" of actual surface contact formed by surface topography, would be violated. Furthermore, the method of Bryant, et. al., if applied to disk drive technology, would require that a "force actuator", which is "coupled" with either the HGA or disk, would accomplish high frequency excitation. Such coupling would, in general, change system properties, due to the low mass of the recording head and HGA. This is clearly undesirable for a representative wear acceleration test method, and accordingly, it was not proposed by the inventors for such a use. Conversely, the de coupling of excitation source and interface behavior, as in the present invention, allows easier tuning of excitation frequency for coincidence with gas bearing resonance.
The patent of Chikazawa, et. al. (U.S. Pat. No. 5,313,352) covers fabrication of recording heads with a plurality of built-in piezoelectric actuators, each excited by an alternating electrical current, so as to vibrate the recording head, and thereby reduce static friction at the head disk interface during device starting and stopping. According to this purpose, in the method proposed by Chikazawa, et. al., the excitation is switched off during normal operation. However, this would cause any wear test, which is based on vibration excitation by such a method, to be insensitive to the me, dial characteristics of the gas bearing present under operating conditions in the disk drive device. Furthermore, such built-in piezoelectric actuators would require the connection of extra wires to each magnetic recording head, consequently hindering any wear testing process that uses the actuators. The presence of these wires would also, in general, change the static torque applied to the head during device operation, and/or change the net torsional stiffness of the head mounting gimbal. This would, in general, corrupt test results if such an actuator equipped head were used for wear testing. The use of such piezoelectric actuators, fabricated as part of the magnetic recording head, is also impractical for accelerated wear testing in a manufacturing environment, since at least one actuator equipped head would have to be specially fabricated for each wear test, and such specially fabricated heads would, in general, not have the same properties as randomly selected representative heads from typical manufacturing lots.